The field of the invention is that of MOSFET integrated circuit processing with field effect transistors having short channel widths and also having excellent punchthrough characteristics, and which can be realized with a VLSI manufacturable process.
In order to fabricate future complex integrated circuits, the basic building block of integrated circuits, the transistor, must become smaller. Smaller metal oxide semiconductor (MOS) transistors are formed by decreasing the channel length of the transistor. Future MOS transistors will have channel lengths of less than 30 nm.
Those skilled in the art are aware of a number of problems that become more difficult is the channel length decreases, referred to generally as short channel effects.
A particular problem in transistor fabrication is the ability to adjust the threshold Vt and also the on-resistance (series resistance) of the transistor. In conventional practice, the two are linked, so that it is not possible to set the values of the two quantities independently.
Conventionally, the halo implant is performed with the wafer tilted so that the implanted ions penetrate underneath the gate. In practice, this method increases the capacitance of the device when the diffusion of the extension implant is greater than estimated and also affects the series resistance of the device.
In particular, a problem with manufacturing such small channel devices is that the punchthrough voltage of these transistors decreases to an unacceptable level.
The punchthrough voltage of a device is the drain voltage that causes the drain depletion region of the device to extend into the source depletion region. When this occurs the transistor conducts regardless of the gate voltage. This eliminates the ability of the transistor to act as a switch, i.e. to switch “on” and “off”. MOS transistors of less than 400 nm gate length cannot be fabricated without adjusting to some degree the process recipe to raise the punchthrough voltage of the device.
Presently the main technique for adjusting the punchthrough voltage and threshold voltage of short channel MOSFET transistors is the halo implant, in which a second implant of the same polarity of the transistor body is made to increase the dopant concentration at the edge of the source and drain and thereby reduce the depletion region. These implants are often made with the wafer oriented at a large tilt with respect to the implanting ions. This implant forms higher concentration P type regions (using an NFET as an example unless otherwise stated) under the gate to prevent bulk punchthrough. This implant is generated by tilting and rotating the wafer as the implant occurs. The dosage is small enough compared with the dose in the source and drain that they are not affected. The gate acts to block the halo dose from reaching the bulk of the transistor body and confines it to the edge of the body in a small region near the low-doped (LDD) region of the source and drain.
Halo transistors exhibit several undesirable features. First, the P implants do not surround the entire drain. This requires wells to be deeper to prevent punchthrough leading to a reduction in packing densities. Second, the doping uniformity is dependent on the placement, shape, and layout of the fabricated transistor, since the implanted ions will be blocked by neighboring structures. As dimensions shrink, the aspect ratio of the gap between neighboring devices increases and the degree of blocking the implant also increases. Additionally, the halo technique requires very specialized, and expensive equipment which increase the cost of applying the method.
Thus, what is needed is a reliable submicron transistor which exhibits excellent punchthrough characteristics without sacrificing other device performance characteristics and which can be fabricated with a VLSI manufacturable process.